This invention relates to a passive ice system and more particularly to a system using radiative cooling effective at temperatures below the dewpoint of the ambient air.
The use of seasonal ice storage for space cooling in industrial buildings and other structures is well known and has been recently emphasized in the disclosures of U.S. Pat. Nos. 4,271,681 and 4,355,522 which are incorporated herein by reference. In general, these disclosures relate to the use of cold air to make ice in a passive ice system using heat pipes and an insulated tank.
One limitation of this passive ice system is that ice formation essentially stops when ambient air temperatures are above the freezing temperature. This limitation can be partially avoided by using a substance different from water as the storage medium. An ice clathrate such as a mixture of water and freon has a freezing point that is higher than the 32.degree. F. of pure water, with the actual temperature depending on the percentage ratio of the components. With selected clathrates, freezing temperatures in the order of 2.degree.-5.degree. F. above 32.degree. F. are obtainable. However, with the conventional cooling system for ice formation, it remains important that the ambient air be at a temperature below the freezing point of the ice-forming composition.
One alternate to the limitation of ambient air temperature is to use radiative cooling. As disclosed in U.S. Pat. Nos. 3,043,112 and 3,310,102, radiative cooling has been used for direct space cooling which have not generally involved ice formation.
More particularly, the cooling systems of these references involve the rejection of heat to the 3 K (3.degree. K.) environment of outer space mainly in the spectral region between about 8-13 microns which may be referred to as the main infrared "atmospheric window". Another window is in the wavelength region between about 19-22 microns. Selective surfaces are provided which are transparent in the region of one or more of the atmospheric windows. While these systems have advantages for direct space cooling, once the surface in contact with ambient air cools to a temperature below the dewpoint of the air, water condenses on the cold surface and effectively blocks the transmission through the window.
Accordingly, one object of the invention is a passive ice system with cooling surfaces effective at lower temperatures. A second object of the invention is a passive ice system useful for longer periods of time in making ice. Another object of the invention is a passive ice system useful in warmer climates. A further object of the invention is a passive ice system utilizing radiative cooling. Yet another object of the invention is a passive ice system utilizing radiative cooling dependent on the atmospheric window. These and other objects of the invention will become apparent from the following description.